Older internal combustion engines, having valve opening and closing occurring at the same times during the engine cycle in dependence on engine-driven cams, had the drawback that the fixed valve opening and closing times (as well as the valve lift profiles, i.e., the degree of lift over time) were optimized for a certain speed range. At this speed range, fuel efficiency, emissions, and power would be optimized, but at other speeds the balance between these factors would vary: for example, an engine designed to have its cams actuate the valves for some preferred fuel efficiency, emissions, and power at high speed might have poor fuel efficiency, emissions, and power at low speed.
This led to the advent of variable valve actuation systems, which would modify the valve profile (the valve position over the engine cycle, i.e., over a 720 degree rotation of the crankshaft) to attain better performance over a wider range of speeds. The simplest variable valve actuation systems simply modify standard cam-based systems to advance or retard the timing of valve openings and closings; more complex systems might independently actuate each valve with solenoids, hydraulic actuators, or the like to allow totally independent control of the timing and lift height of each valve.
However, greater freedom in valve actuation bears greater complexity with engine control strategies. To illustrate, an engine's performance is heavily dependent on the mass air per cylinder (MAC, i.e., the mass of air in the combustion chamber immediately prior to ignition), which is in turn dependent on the valve profile (valve timing and/or lift). Knowledge of the correct MAC is critical for determining the desired amount of fuel to inject or otherwise provide to the combustion chamber (i.e., to get the correct air/fuel ratio), and is also useful for determining spark timing and other engine control parameters. Usually, the MAC is calculated from the “Speed Density” model, wherein the measured air pressure in the intake manifold (Manifold Absolute Pressure or MAP) and temperature are used to calculate a theoretical MAC using ideal gas laws. A volumetric efficiency (VE) correction is then used to compensate for differences between the theoretical and actual MAC (with volumetric efficiency being a measure of the efficiency with which the engine can move the charge into and out of the cylinder, usually expressed as the ratio of the actual flow into the engine as compared to the theoretical flow). Since volumetric efficiency varies with engine speed and load, look-up tables in the Engine Control Unit (ECU, the computer/processor used to control the engine) are usually used to identify the volumetric efficiency at a particular speed and load, and thereby determine the MAC (and thus the injected fuel amount, spark advance, etc.). Since the combustion cycles of different cylinders are usually out of phase (i.e., intake, compression, expansion and exhaust occur at different times in different cylinders), it should be understood that MAC may be calculated at different times during the engine cycle for different cylinders. Thus, in essence, common engine control systems for multi-cylinder engines simply monitor the MAP, have the ECU determine the VE at the current engine speed, and calculate the MAC from the VE and the MAP, with each cylinder undergoing an intake stroke being assumed to receive the same amount of air (i.e., each cylinder having open intake valves is assumed to have the same “average” MAC). Further corrections to the calculated MAC may also be applied, for example, by monitoring exhaust oxygen and adapting the injected fuel to attain the desired air/fuel ratio for the calculated MAC.
However, the speed density model has several drawbacks. Initially, since changes in valve timing and lift also change engine parameters such as volumetric efficiency, it becomes difficult and burdensome to generate look-up tables for all possible valve states. The difficulty and burden is further enhanced when good performance is desired under transient engine conditions (i.e., when the engine is moving between different speeds and loads), since VE may be different under transient conditions than at steady-state operation. The end effect is that the MAC calculated from the MAP is less accurate than it ideally could be, particularly during transient engine conditions, and thus dependent events such as fuel injection amounts and timing, spark timing, etc. are nonideal as well. This in turn results in lost performance, wasted fuel, and/or greater emissions. Additionally, it is often incorrect to assume that the same amount of air is supplied from the manifold to all cylinders which are simultaneously undergoing intake. Different cylinders often have different gas dynamics depending on the manifold and intake runner configuration, engine speed, etc., and while the difference between cylinders is often minor, these minor differences lead to significant performance loss, fuel waste, and excess emissions, since cylinders having a MAC which deviates from the average are effectively being mis-operated.
Other control systems attempt to determine MAC more directly by using a mass air flow (MAF) sensor to determine the air supplied to the cylinders. Usually this arrangement takes the form of an element upstream from the throttle which is heated to a constant temperature, and the current needed to maintain the element at the desired temperature provides a measure of the airflow (which cools the element in accordance with the mass flowing past the element). These systems are also susceptible to error during transient conditions, particularly where sudden changes in throttle position occur. As an example, if the throttle is suddenly opened, a MAF sensor may detect a large surge of air entering the throttle, with the surge arising from air rushing past the throttle and into the manifold (and compressing the air therein). However, the cylinders do not take in all of this air, and thus the MAF sensor's reading leads to an inappropriately large MAC estimate and a correspondingly excessive fuel pulse. Similarly, when the throttle is suddenly closed, the cylinders can draw more air than the MAF sensor measures, leading to an erroneously low MAC estimate (and insufficient fuel injection). Thus, MAF-based injection schemes are also imperfect. Further, MAF-based systems also assume that the quantity of air supplied to each cylinder is equal over an engine cycle—in other words, it is assumed that MAC=MAF*cycle time/number of cylinders. Still other systems measure both MAF and MAP, and use both to determine MAC.
It might be assumed that at least some of the foregoing problems—those regarding the use of average estimated cylinder MACs—might be addressed by measuring MAP and/or MAF to each cylinder individually, as by placing pressure and/or mass flow sensors in individual cylinder intake runners. However, this is generally not practical owing to cost and space constraints, and the periodic behavior of the gas in the runners (as intake valves open and close) makes it very difficult to practically and economically monitor pressure and/or mass flow in an accurate and reliable manner. In contrast, placing MAP sensors in the manifold and/or MAF sensors upstream from the throttle, where the gas flow is more uniform, makes it far easier to monitor pressure and/or mass flow.
It would therefore be beneficial to have an engine control system which address the foregoing problems with prior control systems.